Heavy power devices which output great work or are capable to move heavy objects are very common, especially at construction sites or steel workshops, such as a rear-dump truck or hydraulic crane track. To obtain a great power output, most of the heavy power devices are operated hydraulically. A hydraulic power system requires a pump to pressurize the hydraulic fluid and thus supply the high pressure hydraulic fluid that is needed in operating the hydraulic power device. The pump may be driven by means of an electrical motor or an engine. The pump has to be turned on before the hydraulic power device is operated or the pump has to maintain operating in order to supply the high pressure hydraulic fluid. The pump has to be stopped once the supply of high pressure hydraulic fluid is no longer needed and this may be done by means of for example a clutch or the like coupled between the pump and the motor/engine.
In a regular hydraulically operated device, a controller is provided for the operator to control the supply of the high pressure hydraulic fluid and the moving direction of the hydraulic device. Such a controller may be electrically or pneumatically operated. For certain hydraulic power devices, pneumatic power is more readily available for control purpose, such as a rear-dump truck which itself is equipped with an air compressor or similar device. In such a case, a pneumatic control assembly has advantages over the electrically operated controller.
The control assembly of a hydraulic power device usually comprises two parts, one of which controls the supply of the hydraulic fluid and the other controls the moving direction of the hydraulic power device. It often happens that when the operator switches the direction control to the retracting direction to move the hydraulic power device in the returning stroke which in certain cases requires no supply of the hydraulic fluid, he or she inadvertently leaves the power control in the engaged position which makes the pump to continue supplying the hydraulic fluid to the hydraulic power device. In such a case, damage to the hydraulic power device may occur or even worse, the hydraulic power device may be accidentally actuated and thus causing property and live casualty/damage.
To overcome such a problem, devices that couple the power control of a pneumatic control assembly to the direction control, especially in moving the hydraulic power device in the returning stroke, are available in the market. However, such devices are very complicated in construction and hard to maintain and thus are impractical for the environments of for example the construction site. Thus, it is desired to have a device for coupling the power control to the direction control which is simple in construction and thus cheap in cost and easy in maintenance.